Document WO 99/46055 discloses such a device in which a spherical closure element, which closes the outlet of the reservoir, is expelled by the flow of air created by an air expeller. In order to use a dispenser device more particularly for dispensing powder, the air pressure necessary for actuating the device must be sufficiently high to guarantee that the dose is dispensed completely, and that it is broken up, if that is necessary. In the above-mentioned device, the air pressure necessary to actuate the device is determined by the resistance opposed by the ball in order to be expelled. That resistance is relatively difficult to control and to predetermine since it depends on the friction between the ball and its cylindrical seat in which it is engaged for the purpose of closing said reservoir in leaktight manner. Consequently, it may be necessary to minimize the interference between the sphere and its cylindrical seat, and obviously that might spoil the effectiveness of the closure. Furthermore, it may be necessary to minimize the depth and the positioning of the sphere in its seat so as to make it easier to expel. It may also be necessary to provide air pressure that is relatively high, which is not always easy to achieve by means of a pump system or of a bellows system, in particular when the air expellers are actuated manually by the patient. Furthermore, dispensing, i.e. expelling the ball from its seat, may take place at different positions along the stroke of the pump or of the bellows of the air expeller, such that the precise moment of dispensing the composition cannot always be predetermined in exact manner. Finally, there is a limit on the materials that can be chosen for the sphere and for its seat.
Document WO 02/45866 describes a device in which a closure ball is expelled mechanically by a rod that is secured to an air expeller. That embodiment presents several drawbacks. Thus, an air expeller that is airtight in any position may, in particular during transport or storage, present a risk of said air expeller being triggered in unwanted manner in the event of there being a difference in pressure between the inside and the outside of the air expeller. Furthermore, the device in document WO 02/45866 is not reusable, and the entire device must be thrown away after it has been used. In particular, for ecological and economic reasons, it may be desirable to have a reusable device in which the reservoir is changed after each actuation, but not the air expeller.
Document FR 2 186 853 describes another prior-art device.